Pacific Gas and Electric Co. officials said Thursday that the utility has repeatedly boosted the pressure on several urban natural gas lines that have what the federal government has classified as substandard welds.

Experts said the practice needlessly stressed lines with features that have been tied to scores of pipeline failures nationwide.

The utility said the pressure boosts were legal, technically sound and safe, and were intended to preserve its ability to deliver gas on cold days. But critics say the spiked lines contained substandard welds common in pre-1970s pipe that were at obvious risk of sudden failure.

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The federal government warned pipeline companies in the late 1980s to reconsider use of lines with low-frequency electric resistance welds, after studies had linked such welds to more than 100 pipeline ruptures and other failures.

Robert Eiber, a pipeline integrity expert with 50 years' experience, said PG&E's spikings were "the dumbest thing they could do."

"They are playing games with public safety," Eiber said. "They got caught. Now they are going to live with the banner of the federal regulators or the California PUC. They are going to have to bring their system up to snuff."

Miles of older line

More than two-thirds of PG&E's 5,700 miles of gas transmission lines consist of pipes that predate 1970. Rather than embark on a widespread replacement program after the federal advisory, PG&E intentionally boosted pressure to the legal maximum temporarily on lines subject to a 2002 inspection law.

By doing so, it avoided the risk of having an accidental pressure spike force it to conduct expensive, time-consuming tests that could detect pipeline weaknesses.

PG&E contends that federal law governing pipelines with electric resistance welds "requires them to increase the pressure to re-establish their maximum operating pressure every five years," Paul Clanon, executive director of the state Public Utilities Commission, told Assemblyman Jerry Hill, D-San Mateo, in a letter dated Feb. 7.

Until now, PG&E had said that it boosted pressure only to maintain "operational flexibility" on the lines. It had not previously said it knew the transmission lines had the pre-1970 welds, or that it had spiked those lines specifically because they had those welds.

Federal law

Kirk Johnson, PG&E's senior vice president for gas engineering, confirmed Thursday that the utility believed it had to raise the pressure on such lines to its legal maximum at least once every five years, or risk having to run lines at lower pressures under a federal law intended to cut the risk from the problematic older welds.

"There is a segment of the code that states clearly that a pipeline, certain types of pipelines, certain segments of pipelines ... cannot operate or should not operate at a pressure that is higher than is seen in the previous five years," Johnson said.

He said such pipelines are defined as ones that have welds that are weaker than the pipe wall itself. That definition includes pipes with the pre-1970 electric resistance welds, he said.

Bay Area lines

In the Bay Area, lines with the vulnerable welds that PG&E spiked since 2003 include two running along the Peninsula into San Francisco. One was Line 132, the San Bruno pipeline that exploded in September, killing eight people and destroying 38 homes.

They also include one running from Fremont to Livermore that PG&E has listed as its "highest risk" transmission line in the Bay Area.

Several experts said boosting the pressure on such notoriously vulnerable lines put the public at risk.

"It shows an incredible recklessness in understanding of the risk of these pipe welds - you don't just go out and wham this stuff, because it has a historical risk of rupture failure," said Richard Kuprewicz, a pipeline safety expert in Redmond, Wash.

PG&E officials maintained that its practice was not dangerous because the legal pressure levels have a large margin for error.

"It was a safe operation," Johnson said. The utility halted the practice, at least temporarily, after the San Bruno disaster.

Inspection triggers

Under the federal rules, gas transmission lines in urban areas that have the older welds are immune from expensive inspections if pressure is kept below a certain level.

That level, in effect, is set by the utility, because it is defined as the highest pressure at which the line has run over a five-year period. If PG&E didn't boost an older line's pressure as high as legally allowed, Johnson said, the utility would have to set the cap at whatever the five-year peak pressure was.

If the pressure accidentally goes beyond that five-year high point, the utility has to conduct a costly and burdensome weld-defect inspection, either by running an automated device, known as a smart pig , through the pipe or by shutting down the line temporarily and filling it with high-pressure water.

On the San Bruno line, PG&E spiked the pressure to just above the pipe's legal limit of 400 pounds per square inch in December 2003 and again in December 2008. A utility spokesman recently acknowledged that a portion of the line between Milpitas and Daly City, outside the San Bruno area, had an electric resistance weld.

Electric resistance welds are also found on Line 109 running into San Francisco, which PG&E spiked twice, and on Line 107 from Fremont to Livermore.

The National Transportation Safety Board, which is investigating the San Bruno disaster, says the explosion started at a different type of seam weld that penetrated only half the pipe wall. Experts say the board's photos of the weld showed stress marks caused by pressure surges over the years.

Johnson downplayed any risk to spiking a pipe to or even above the legal limit, saying federal law governing pressure levels provides an ample safety protection.

UC Berkeley engineering Professor Bob Bea, however, said the practice "does not make sense."

"This goes back to their belief that they had no significant chance of failure, and that set off a series of illogical steps that shows up in the saga of San Bruno," Bea said.

Kuprewicz said federal law was clearly intended to make sure that the problematic welds were not stressed.

"PG&E's interpretation is a violation of the laws of science regarding (electric resistance) welds," Kuprewicz said. "It's creating a more dangerous environment for the public to do the one thing, if you understand how an (electric resistance weld) fails, you would not want to do."

Spiked lines

January

-- PG&E confirmed that it had boosted pressure to the maximum legal limit in December 2008 on the San Bruno pipeline that exploded 21 months later. It later revealed that it had also pushed gas levels to the legal limit in December 2003.

-- A day after The Chronicle reported the 2008 spiking, PG&E said it had halted the practice. It said it had been boosting pressure to maintain "operational flexibility."

-- Two weeks later, PG&E said it had conducted similar pressure surges in recent years on 10 other transmission lines, but refused to say where.

February

-- PG&E revealed the lines' locations. They included pipes running into San Francisco and one from Fremont to Livermore that the utility considered to be its transmission line at "highest risk" of failure in the Bay Area.

-- The utility revealed that pressures had spiked accidentally on four other natural-gas transmission system lines to well above legal maximums. State regulators ordered the utility to reduce pressure on those pipes immediately, fearing the spikes could have weakened the lines.

-- This week, PG&E said several of the transmission lines it spiked intentionally had pre-1970 welds that federal officials have identified as being at a high risk of failure. The utility said it had done so to avoid having to drop its maximum pressure limits on the lines.